1. Field of the Invention
The present invention relates to an acceleration sensor configured to detect, in a state where a weight member is connected to a vibrating portion and the vibrating portion is vibrated at a resonant frequency of a natural vibration thereof, a magnitude of acceleration from a change in the resonant frequency of the natural vibration of the vibrating portion, which change is generated upon application of acceleration to the weight member.
2. Description of the Related Art
Examples of structures of related-art acceleration sensors will be described below. An acceleration sensor according to a first related art (see, e.g., Japanese Unexamined Patent Application Publication No. 2006-308291) includes an acceleration sensor device 101 and a control circuit (not illustrated). FIG. 10A is a plan view, looking at an X-Y plane, of the acceleration sensor device 101 that constitutes the acceleration sensor according to the first related art. The acceleration sensor device 101 includes a frame 102, holding members 103A, 103B and 103C, a support member 104, and a vibrating plate 105. In the following description, an axis extending in the lengthwise direction of the vibrating plate 105 of the acceleration sensor device 101 is defined as an X-axis of an orthogonal coordinate system, an axis extending in the direction normal to the vibrating plate 105 (i.e., the direction of thickness thereof) is defined as a Z-axis of the orthogonal coordinate system, and an axis being perpendicular to the X-axis and the Z-axis is defined as a Y-axis of the orthogonal coordinate system.
The frame 102 has a frame-like shape. The holding members 103A, 103B and 103C, the support member 104, and the vibrating plate 105 are arranged inside the frame. Led-out electrodes 116A and 116B are disposed on the frame 102. The frame 102 holds the holding members 103A, 103B and 103C, and the vibrating plate 105. The holding members 103A, 103B and 103C hold the support member 104. The support member 104 supports the vibrating plate 105 in cooperation with the frame 102. The support member 104 functions as a weight member.
The vibrating plate 105 is in the form of a beam. The vibrating plate 105 is connected to the frame 102 at a base portion 106A thereof, i.e., at one end portion of the vibrating plate 105 in the X-axis direction, and is connected to the support member 104 at a base portion 106B thereof, i.e., at the other end portion of the vibrating plate 105 in the X-axis direction.
The support member 104 is connected to the frame 102 through the holding members 103A, 103B and 103C. In more detail, the support member 104 is supported by the frame 102 through the holding members 103A, 103B and 103C at two positions on the left side and at one position on the right side when viewed in the drawing sheet of FIG. 10A. Each of the holding members 103A, 103B and 103C is in the form of a beam and has a spring structure (meander structure) in which the beam is folded several times in a zigzag manner alternately in opposite directions with respect to the X-axis direction. Thus, the support member 104 is supported to be reciprocally movable only in the X-axis direction.
FIG. 10B is a perspective view illustrating, in enlarged scale, the vibrating plate 105 of the acceleration sensor device 101 that constitutes the acceleration sensor according to the first related art. The vibrating plate 105 is constituted by a silicon (Si) layer 112 formed on a silicon dioxide (SiO2) layer 111, a lower electrode layer 113 formed on the Si layer 112, a piezoelectric thin film layer 114 formed on the lower electrode layer 113, and an upper electrode layer (115A and 115B) formed on the piezoelectric thin film layer 114. The upper electrode layer is made up of a detection electrode 115A and a driving electrode 115B. The detection electrode 115A is formed to extend over not only a region spanning from substantially a center of the vibrating plate 105 in the lengthwise direction to an end portion thereof on the side including the base portion 106A, but also over a region near a portion of the frame 102 where the frame 102 is connected to the base portion 106A of the vibrating plate 105. The detection electrode 115A is connected to the led-out electrode 116A. The driving electrode 115B is formed to extend over not only a region spanning from substantially the center of the vibrating plate 105 in the lengthwise direction to an end portion thereof on the side including the base portion 106B, but also over the support member 104, the holding member 103C, and a region near a portion of the frame 102 where the frame 102 is connected to the holding member 103C. The detection electrode 115B is connected to the led-out electrode 116B. The led-out electrodes 116A and 116B are connected to a control circuit.
In the acceleration sensor according to the first related art, when a driving signal is input to the led-out electrode 116B from the control circuit, a portion of the piezoelectric thin film layer 114, the portion being located in a region where the driving electrode 115B and the lower electrode layer 113 are opposed to each other, is caused to extend and contract upon application of an electric field generated by the driving signal. The vibrating plate 105 is thus vibrated. At that time, with the vibration of the vibrating plate 105, pressure is exerted on the portion of the piezoelectric thin film layer 114, located in the region where the detection electrode 115A and the lower electrode layer 113 are opposed to each other, whereby electric charges are generated in that portion. The generated electric charges are output as a detection signal from the led-out electrode 116A.
By using the detection signal, the control circuit drives the acceleration sensor device 101 into a state where the vibrating plate 105 is driven and vibrated stably at a resonant frequency of the natural vibration thereof.
When acceleration in the X-axis direction is applied, as denoted by an arrow G, to the acceleration sensor according to the first related art in the state where the vibrating plate 105 is driven and vibrated, the support member 104 is displaced in the X-axis direction by an inertial force generated upon the application of the acceleration. Accordingly, the vibrating plate 105 in the driven and vibrated state is caused to extend (or contract) in the X-axis direction by a force acting on the vibrating plate 105 from the support member 104 with the displacement of the support member 104, and the resonant frequency of the natural vibration of the vibrating plate 105 is changed. Thus, a frequency of the detection signal is changed in accordance with the change in the resonant frequency of the natural vibration of the vibrating plate 105, and a magnitude of the acceleration can be detected from the frequency change of the detection signal.
There is also an acceleration sensor of optical detection type.
FIG. 11A is a plan view, looking at an X-Y plane, of an acceleration sensor device 201 according to second related art (see, e.g., Japanese Unexamined Patent Application Publication No. 2006-105764). FIG. 11B is a side view, looking at a Y-Z plane, of the acceleration sensor device 201.
The acceleration sensor device 201 includes fixation members 202A and 202B, coupling members 203A, 203B, 203C and 203D, a weight member 204, a light source 205, and a light detector 206. In the following description, an axis extending along the direction in which the fixation member 202A, the coupling members 203A and 203B, the weight member 204, the coupling members 203C and 203D, and the fixation member 202B are arranged in the acceleration sensor device 201 in the mentioned order is defined as an X-axis of an orthogonal coordinate system, an axis extending in the direction normal to the weight member 204 (i.e., the direction of thickness thereof) is defined as a Z-axis of the orthogonal coordinate system, and an axis being perpendicular to the X-axis and the Z-axis is defined as a Y-axis of the orthogonal coordinate system.
As illustrated in FIG. 11A, the fixation members 202A and 202B are arranged on both sides of the weight member 204 in the X-axis direction. The fixation member 202A is connected to the weight member 204 by the coupling members 203A and 203B. The fixation member 202B is connected to the weight member 204 by the coupling members 203C and 203D. The weight member 204 preferably is in the form of a quadrangular plate having two sides parallel to the X-axis and two sides parallel to the Y-axis when looked at in a plan view. The weight member 204 is supported by the fixation members 202A and 202B on both the sides through the coupling members 203A, 203B, 203C and 203D.
Each of the coupling members 203A, 203B, 203C and 203D has two bent portions 211 that are bent with respect to the X-axis direction. The bent portions 211 of each of the coupling members 203A, 203B, 203C and 203D provide resiliency allowing each coupling member to extend and contract in the X-axis direction. Accordingly, the weight member 204 supported by the fixation members 202A and 202B on both the sides through the coupling members 203A, 203B, 203C and 203D is smoothly displaceable in the X-axis direction.
As illustrated in FIG. 11B, the light source 205 is fixed above the weight member 204. The light detector 206 is fixed under the weight member 204. Light emitted from the light source 205 is partly blocked off by the weight member 204, and the remaining part of the emitted light is received by the light detector 206 without being blocked off by the weight member 204. The light detector 206 outputs a current corresponding to an amount of the received light.
Because of the weight member 204 being smoothly displaceable in the X-axis direction, even when acceleration in the X-axis direction is slightly applied to the acceleration sensor 201, the weight member 204 is displaced sufficiently, thereby causing sufficient change in the amount of light received by the light detector 206. Hence a magnitude of the output current from the light detector 206 is changed. Therefore, the presence or the absence of acceleration and the magnitude of the acceleration can be detected by measuring the magnitude of the output current from the light detector 206 with an electronic circuit (not illustrated). Furthermore, since the weight member 204 is coupled to the fixation members 202A and 202B by the four coupling members 203A, 203B, 203C and 203D in total, the weight member 204 is supported with sufficient strength, and the strength of the acceleration sensor is increased in its entirety.
In the above-described acceleration sensor device 101, the support member 104 is connected to the holding members 103A, 103B and 103C at two positions on the left side and at one position on the right side, whereas the frame 102 is connected to the holding members 103A, 103B and 103C at two positions on the left side and at two positions on the right side. In the above-described acceleration sensor 201, the weight member 204 is connected to the coupling members 203A, 203B, 203C and 203D at two positions on the left side and at two positions on the right side, whereas the fixation members 202A and 202B are connected to the coupling members 203A, 203B, 203C and 203D at two positions on the left side and at two positions on the right side.
When the weight member is supported at many positions as in the above-described cases, the inertial force generated upon the application of acceleration is distributed and a displacement magnitude of the weight member is reduced. Hence sensitivity of detecting the acceleration in the acceleration sensor is reduced. Moreover, in the case of detecting the acceleration with the light detector, the sensor size in the Z-axis direction is increased and reduction in size is difficult to realize.